Magnesium alloys are a strong and light weight alternative to traditional alloys, such as aluminum alloys. Although magnesium has a higher strength to weight ratio than aluminum, castings made from magnesium typically suffer from porosity, which compromises casting strength. As a result, many dies are currently designed to produce castings with features such as thicker ribs and wall sections, thus eliminating weight savings realized through the use of magnesium and making the finished part unacceptably high in cost. In order to increase utilization of magnesium in die casting applications, improved casting processes to increase the strength of the finished parts and reduce scrap costs are needed.
Magnesium die castings are typically made using techniques developed for aluminum die casting. These techniques generally are unsuitable when applied to magnesium die casting. One such technique relies on vacuum to transfer liquid alloy from the casting furnace to the die casting machine for subsequent injection into the die. Typically, a vacuum is used to evacuate the die cavity and, to draw the metal from the casting furnace into the die casting machine. In a die casting machine, a shot sleeve is normally provided for injection of the molten metal. The shot sleeve is a tube of sufficient volume to hold the full charge of molten metal needed to fill the die and the passages leading up to the die, called runners. The shot sleeve is normally evacuated along with the die cavity and molten metal flows into the shot sleeve from the top.
Filling the shot sleeve using a vacuum is difficult with magnesium because of the much higher filling temperature as compared with aluminum. The high temperature makes it difficult to maintain a vacuum seal and lead to excessive equipment maintenance. Also, since the cycle time between each shot sleeve filling operation is quite long, in excess of ten seconds, and since magnesium cools at a rate approximately three times faster than aluminum, the magnesium has a tendency to solidify within the shot sleeve and runners. This prevents pressure intensification of the molten casting. Pressure intensification is performed by the shot plunger immediately after injection, reducing casting porosity and causing the casting grain structure to be altered from dendridic to the more desirable globular grain structure, thereby improving the mechanical properties of the finished casting. Solidification prevents rapid pressure intensification, leading to poor part quality and large numbers of scrap castings. The long cycle times also do not permit economically acceptable production rates.
The manner in which the shot sleeve is filled also affects the quality of magnesium die castings. Improper filling of the shot sleeve can cause non-uniform metal flow, entrapment of gases that cause casting porosity, and segregation of impurities, such as magnesium oxide. These factors detrimentally affect the quality of the castings produced.
Research conducted with aluminum has shown that filling the shot sleeve from the bottom in combination with a vacuum can reduce some of these common problems. However, vacuum methods commonly used with aluminum cannot be readily applied to magnesium. The need therefore still exists for an improved magnesium die casting system to improve casting strength and reduce the number of scrap castings.
A pump, such as the one disclosed in U.S. Pat. No. 6,602,462, issued Aug. 5, 2003, which is hereby incorporated by reference, may be used to transfer molten magnesium from the melt furnace to the shot sleeve instead of a vacuum.
Magnesium die casting by nature produces a high percentage of scrap metal, typically 40 to 60% of the total shot weight. This can largely be attributed to the extensive runner system required to completely fill the die, which becomes scrap once the part is cast. In order to reduce material costs, it is desirable to recycle scrap magnesium. Magnesium re-cycling for automotive applications is currently done off-sit by approved recyclers in order to ensure final product purity. It is expensive to have magnesium recycled in this manner due to recycling costs, increased inventory requirements, and handling costs.
Scrap magnesium is sometimes recycled using in-house facilities. Typically, scrap magnesium metal is recycled using a flux based process, wherein surface metal oxides are wetted and agglomerate into globules of flux. The globules or spheres settle to the bottom of the melt and are removed as sludge from a bottom region. However, during globule formation, liquid metal is entrapped within the globules, resulting in a loss of recyclable material. The melt is also sparged with fine bubbles of an inert gas, such as argon. The argon forms a blanket of cover gas in the furnace that prevents contact with air. As a result of sparging, light weight impurities in the recycled material rise to the top of the melt and may be removed from a top region. Pure metal is withdrawn from a clean region between the top and bottom regions in about the center of the furnace.
Withdrawal of sludge from the furnace causes the melt level to drop, requiring a make-up addition of virgin magnesium. The withdrawal and addition of fresh material can lead to temperature fluctuations within the furnace and increased crucible maintenance. It is therefore desirable to reduce the amount of material withdrawn from the furnace as sludge and impurities. Also, flux cannot be introduced into the die castings and flux based processes are typically conducted off-line to minimize the risk of casting contamination. Scrap is cooled, taken to the off-line recycling centre, re-heated in a furnace, then cast into ingots and cooled. The cooled ingots are then introduced into the casting furnace for use in making die castings. The cooling and re-heating of the scrap material consumes a great deal of energy, making off-line flux based recycling processes expensive and impractical.
The need therefore still exists for an improved magnesium die casting system incorporating recycling process.